Blazer Fuel Pressure Won't Hold

I have a 1994 S10 Blazer, 4.3L Vortec, with 142,000 miles on it. I replaced the fuel
pump due to low fuel pressure while running (barely 51 PSI). I was checking for
problems as to why it was stalling and not idling correctly. Pinching off the return
line showed the pressure steady, so that meant the pressure regulator was fine. With
the previous pump, it would build pressure fine for the 2 second run before starting,
and hold at about 60 PSI. The truck started easily all the time, just ran kind of
crappy. After replacing the fuel pump (twice with the same result), it builds pressure
to 60 PSI, and then quickly drops to zero. This now makes the truck very difficult to
start, and with the very cold New England weather recently, only a shot of starting
fluid gets the truck running. I don't want to have to drop the tank again if unnecessary.
The pump I installed was from Advance Auto Parts, and it is a Master Brand E3270
(pictured below) which by all indications is a direct after-market replacement. I had
also replaced the pulse-dampener during the first pump replacement, in the hopes of
only doing this chore once. The test I did before installing the second pump was to
build pressure by turning on the key for the 2 seconds, get the pressure to 60 PSI,
and pinch off the high pressure supply line. I was able to hold the 60 PSI fine. It
then dropped to zero when I released it. There is no leaking fuel anywhere along the
lines, so I don't suspect a defective line, and at 60 PSI i would expect it to be spewing
gas. What am I missing? What holds the pressure for the start? I understood it to be
the fuel pump itself. Should I change to a factory (AC-DELCO?) unit? Thanks in advance!